+1 for alt tabbing, either in Windowed Mode or not. Steam's browser is slow and it takes you out of the game just as much as ALt + Tab into your broswer of choice.

Yup,I only use steam with GW2 for the chat overlay to be able to talk to my friends who aren't on GW2, but playing other games, I have not noticed a performance difference between having it with GW2 and not having it. If I need to check something on the web, I Alt-Tab even with steam.

I actually took the time to have gw2 go through steam just so that steam friends could see when I was on, tried it out and I didn't notice any increased lag or decreased fps... but the sound was about 1/10th of what it normally is. This also happens when I'm talking to people in steam. There's probably a setting that I'm missing to put the sound back up to normal, but I'm not seeing it anywhere.

A friend of mine told me about steam today when I talked about wanting to use a browser in-game. I was wondering if using steam could slow the game down somehow. Maybe effect FPS?

I use Steam with GW2 mainly for chat purposes, and I assure you it won't affect game performance at all. But Steam's in-game browser is a throwback to web browsers of the late 1990's. Its featureless (aside from basic tabs), slow and clunky to use, and I find it has rendering problems on some sites. But to answer your question, no, it works fine if none of the above bothers you.